Ultrasonic examinations are performed within the nuclear industry and other industries to determine the condition of parts and components. The metal or alloy material of a part or component is inspected using ultrasound to detect any flaws which could prove detrimental to the safe operation of that part or component. The ultrasonic NDE method can be used to detect internal flaws in most engineering metals and alloys. Bonds produced by welding, brazing, soldering and adhesive bonding can also be ultrasonically inspected.
Ultrasonic inspection is used for quality control and materials inspection in the fabrication of structures, reactor pressure vessels, airframes, pipe systems, bridges, motor vehicles and jet engines. The present invention has application in all of these fields.
For successful application of ultrasonic examination techniques, the ultrasonic system, including transducers, must be suitable for the type of inspection being performed. If the proper transducer is not used, there is a high potential for gross error in the inspection results, or there could be no results at all. For instance, using a common ultrasonic transducer that has a hard flat-surfaced Lucite wedge for examining as-welded overlaid pipe welds causes gross errors in the ultrasonic inspection results. In many cases ultrasonic inspection data is not recorded at all. This is due to the presence of air gaps between the transducer head and the rough surface being inspected, which air gaps form an opaque barrier.
Ultrasonic characterization of cracks in materials is at least a two-step process: 1) detection and location; and 2) sizing in absolute or relative terms. In accordance with the first step of this process, the transducer is excited to emit an ultrasonic wave which is coupled to the structure being inspected. The emitted wave enters the structure, where it is reflected by the crack. The return path of the reflected wave impinges on the transducer, where it is detected as a "pulse echo" signal.
The determination of the crack size, or depth of penetration in the case of surface-connected flaws, is a different and more complicated task. A conventional method for determining the depth of penetration of a planar crack is the back-scattered time-of-flight technique. This method takes advantage of the backward scattering of waves of ultrasonic energy at the edges of a crack. An emitter of short pulses of ultrasound, coupled to the inspection surface, causes refracted sound waves to impinge on the crack edge, which scatters the ultrasonic energy in all directions. A detector situated on the same or opposite surface as the crack is excited by scattered pulsed energy after a time delay. The time delay is a function of the crack height, the angle of refraction and other dimensions. By measuring the time-of-flight for the round trip from the transducer to the crack edge and back to the transducer, the crack height can be easily computed from the geometry.
Such ultrasonic inspections of the structural integrity of industrial components made of steel and other metals depend upon knowing the beam profiles of the ultrasonic waves that propagate into these components. It is common practice to control the refracted angle of the ultrasound by using hard shoes that follow the surface and maintain, more or less, a constant angle. Surfaces that are rough, with both short-term and long-term roughness, pose a problem because it is difficult to maintain contact. In addition, anisotropic materials, such as stainless steel weld metal and cast stainless steel components, redirect the ultrasound inside the material in an unpredictable manner.
Rough surface conditions and anisotropic grain structure can result in unpredictable results using conventional examination methods. The angle of refraction of an ultrasonic transducer in a probe having a flat contact surface is measured on a special calibration block, and then it is assumed that the angle is the same when applied to a specimen. The angle of refraction within a given material is controlled by the ultrasonic transducer's angle of incidence, i.e., the number of degrees by which the path of propagation is tilted relative to an axis normal to the object surface. The angle of incidence is determined in accordance with Snell's Law, which can be expressed mathematically as: EQU sin a/sin b=V.sub.1 /V.sub.2
where a is the angle of incidence; b is the angle of refraction; V.sub.1 and V.sub.2 are the respective wave velocities in the first and second media. Snell's Law describes wave behavior at an interface between two different media. The law applies even if mode conversion occurs.
Right circular cylinder type ultrasonic probes are used within the nuclear industry to perform inspections of components such as main recirculation pump shafts, head hold down studs, incore housings, stub tube to control rod drive housings and various other right circular cylinder type components. Standard ultrasonic transducer reference blocks currently used within the ultrasonic industry have flat contact surfaces and are designed for contact transducers that also have flat contact surfaces. Thus, there is a need for a standard ultrasonic transducer reference block which can be used to determine the operating characteristics of ultrasonic transducers contained within right circular cylinder type probes.
Standard reference blocks are used by the nondestructive testing industry mainly to calibrate instruments prior to contact inspection using angle-beam search units. These blocks are also useful for checking the performance of both angle-beam and straight-beam search units and for evaluating instrument performance.
Common ultrasonic reference standards such as the International Institute of Welding (IIW) type 1 block and the miniature angle beam block, a derivative of the IIW block, and various other rectangular or flat reference blocks have flat contact surfaces. These common reference blocks are designed specifically for applying flat-surfaced contact transducers and therefore are not applicable for right circular cylinder type probes that have curved transducer contact surfaces.